"""Shared constants and writers for the .sras test fixtures.

The constants mirror the values tests/gen_goldens.py used when the committed
golden files were generated against the pre-refactor code (commit d185676);
they must never change, or the comparisons against those files stop meaning
anything.  Those goldens are legacy v6 files — the one background per file
layout — and are now read-only fixtures for the parser.

``write_v7`` builds the current layout (one background per angle, inside the
data block) over the same geometry, for the tests that need a file this
version of the app could actually have written.
"""
from core.scan_geometry import build_plan
from core.sras_format import VERSION, create_scan_file, write_background_block

SPF = 8
SAMPLE_RATE = 6.25e9
CHANNELS = [1, 3, 4]
PREAMBLES = [f"WFMOUTPRE:CH{ch};SYNTHETIC;PT_FMT Y;XINCR 1.6E-10" for ch in CHANNELS]
BACKGROUND = bytes(range(SPF))

# build_plan inputs for the fixture geometry: 2 angles × 3 rows × 4 frames
TINY_PLAN_ARGS = dict(x_start=1.0, y_start=1.0, x_delta=0.02, y_delta=0.02,
                      num_angles=2, row_spacing=0.01)
LASER_FREQ_HZ = 20000.0
VELOCITY_MM_S = 100.0


def synthetic_frame(ai, ri, ci, fi):
    return bytes((ai * 7 + ri * 5 + ci * 3 + fi + s) % 256 for s in range(SPF))


def tiny_plan():
    """The fixture geometry: 2 angles × 3 rows × 4 frames."""
    return build_plan(**TINY_PLAN_ARGS, laser_freq_hz=LASER_FREQ_HZ,
                      velocity_mm_s=VELOCITY_MM_S)


def angle_background(ai):
    """A background that differs per angle, so tests can tell them apart."""
    return bytes((ai * 11 + s) % 256 for s in range(SPF))


def write_v7(path, plan=None, version=None):
    """Write a complete current-format file: [background][rows] per angle."""
    plan = plan if plan is not None else tiny_plan()
    f = create_scan_file(path, plan, SPF, SAMPLE_RATE, PREAMBLES,
                         version=VERSION if version is None else version)
    try:
        for ai, pa in enumerate(plan.per_angle):
            write_background_block(f, angle_background(ai))
            for ri in range(pa.n_rows):
                for ci in range(len(CHANNELS)):
                    for fi in range(pa.n_frames):
                        f.write(synthetic_frame(ai, ri, ci, fi))
    finally:
        f.close()
    return plan